When speech/audio signals are transmitted in a packet communication system typified by Internet communication and a mobile communication system, a compression/coding technique is often used to improve the transmission rate of speech/audio signals. Furthermore, in recent years, in addition to a demand for simply encoding speech/audio signals at low bit rates, there is an increasing demand for a technique to encode speech/audio signals in high quality.
To meet this demand, studies are underway to develop various techniques to perform orthogonal transformation (i.e. time-frequency transformation) of a speech signal to extract frequency components (i.e. spectrum) of the speech signal and apply various processing such as linear transformation and non-linear transformation to the calculated spectrum to improve the quality of the decoded signal (see, for example, patent literature 1). According to the method disclosed in patent literature 1, first, a frequency spectrum contained in a speech signal of a certain time length is analyzed, and then non-linear transformation processing to emphasize greater spectrum power values is applied to the analyzed spectrum. Next, linear smoothing processing for the spectrum subjected to non-linear transformation processing, is performed in the frequency domain. After this, inverse non-linear transformation processing is performed to cancel non-linear transformation characteristics, and, furthermore, inverse smoothing processing is performed to cancel smoothing characteristics, so that noise components included in the speech signal over the entire band are suppressed. Thus, with the method disclosed in patent literature 1, all samples of a spectrum acquired from a speech signal are subjected to non-linear transformation processing and then the spectrum is smoothed, so that the speech signal is acquired in good quality. Patent literature 1 introduces transformation methods such as power transform and logarithmic transform as examples of non-linear processing.